Word of the Week: Knee

knee [ nee ] noun: a conical woody growth above water from the roots of some trees

 

Cypress knees at Meadowlark Botanical Gardens. Photo © Elaine Mills

Knees are commonly seen on cypresses growing in the southeastern United States. They have a conical shape almost like a termite mound, but are not caused by insects. They grow on the upper surface of the horizontal roots, formed from the vascular cambium, the layer that produces the phloem and xylem, passageways for the food and water moving through plants. They have been recorded on trees aged 12 or older. Solid when formed, they may become hollow from rot in old age.

The function of cypress knees is not yet well understood, though scientists have speculated and researched them for many years. The French botanist F. J. Michaux wrote in 1819 that “No cause can be assigned for their existence.” The two trees most commonly exhibiting knees in the United States are Taxodium distichum var. distichum (bald cypress) and Taxodium distichum var. imbricarium (pond cypress), both deciduous conifers. Bald cypresses have needle-like leaves arranged in two rows. Pond cypress are generally smaller, and usually have ascending scale-like leaves radially arranged around the branchlets. Bald cypresses grow along the coastal plain from the Mid-Atlantic region south to Florida and west to Texas, as well as inland in the Mississippi Valley. Scientists believe the genus Taxodium has been growing in America for about seventy million years but do not know when or why the knees appeared.

Not all cypresses have knees. Those on dry land generally do not, though it is not unheard of, nor do those growing in deep water. Most frequently they appear on trees in shallow water or water whose level varies with weather or season. The trunks of cypresses also frequently have buttress-like bases and fluting, so some researchers think the knees may also have a stabilizing function in a tree that may be subject to blowing over in strong winds because the wet soil is too soft to hold it. However, some doubt that, as cypresses standing in deep water are also in wet soil and do not have knees. The size of knees varies from quite small to a record 14 feet tall.

Meadowlark Botanical Gardens’ informational sign espouses theories about cypress knees. Photo © Robert Kline

Other theories for the function of cypress knees include the aeration hypothesis, the idea that the knees are a form of pneumatophore, a breathing aerial root that helps provide oxygen to trees growing in wet, poorly aerated soil. While some studies have shown the knees transport oxygen, others have shown they transport less than other tree roots, so again, no definitive research has confirmed this.

There is a methane emissions hypothesis, a theory that knees are there to provide a way of vegetative reproduction, or that they are there to catch nutrients drifting by during times when water is in movement. There has also been research confirming the finding of organisms for storing starch in the knees. So far, none of these hypotheses has been satisfactorily proven and the reasons why cypress trees have knees remains a mystery open to further investigation.

References

baldcypress. Virginia Tech Dendrology Factsheets. Virginia Tech Department of Forest Resources and Environmental Conservation.Virginia Tech Dendrology.

Briand CH. 2000. Cypress knees: An enduring enigma. Arnoldia, 60(4): 19–25.

Martin CE. Francke SK. Logan B (editor). 2015. Root Aeration Function of Baldcypress Knees (Taxodium distichum). International Journal of Plant Sciences, 176(2): 170–173. doi: 10.1086/679618.

Taxodium distichum. Landscape Plants. Oregon State University, College of Agricultural Sciences, Department of Horticulture.

Taxodium distichum. Plant Toolbox. North Carolina State Extension.

Taxodium distichum var. distichum. Plant Finder. Missouri Botanical Garden.

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